Re: epoll and multiple processes - eliminate unneeded process wake-ups

[Jason Baron]

On 08/03/2015 07:48 PM, Eric Wong wrote:
> Madars Vitolins <m@xxxxxxxxxxx> wrote:
>> Hi Folks,
>>
>> I am developing kind of open systems application, which uses
>> multiple processes/executables where each of them monitors some set
>> of resources (in this case POSIX Queues) via epoll interface. For
>> example when 10 processes on same queue are in state of epoll_wait()
>> and one message arrives, all 10 processes gets woken up and all of
>> them tries to read the message from Q. One succeeds, the others gets
>> EAGAIN error. The problem is with those others, which generates
>> extra context switches - useless CPU usage. With more processes
>> inefficiency gets higher.
>>
>> I tried to use EPOLLONESHOT, but no help. Seems this is suitable for
>> multi-threaded application and not for multi-process application.
> 
> Correct.  Most FDs are not shared across processes.
> 
>> Ideal mechanism for this would be:
>> 1. If multiple epoll sets in kernel matches same event and one or
>> more processes are in state of epoll_wait() - then send event only
>> to one waiter.
>> 2. If none of processes are in wait state, then send the event to
>> all epoll sets (as it is currently). Then the first free process
>> will grab the event.
> 
> Jason Baron was working on this (search LKML archives for
> EPOLLEXCLUSIVE, EPOLLROUNDROBIN, EPOLL_ROTATE)
> 
> However, I was unconvinced about modifying epoll.
> 
> Perhaps I may be more easily convinced about your mqueue case than his
> case for listen sockets, though[*]
> 

Yeah, so I implemented an 'EPOLL_ROTATE' mode, where you could have
multiple epoll fds (or epoll sets) attached to the same wakeup source,
and have the wakeups 'rotate' among the epoll sets. The wakeup
essentially walks the list of waiters, wakes up the first thread
that is actively in epoll_wait(), stops and moves the woken up
epoll set to the end of the list. So it attempts to balance
the wakeups among the epoll sets, I think in the way that you
were describing.

Here is the patchset:

https://lkml.org/lkml/2015/2/24/667

The test program shows how to use the API. Essentially, you
have to create a 'dummy' epoll fd with the 'EPOLL_ROTATE' flag,
which you then attach to you're shared wakeup source and
then to your epoll sets. Please let me know if its unclear.

Thanks,

-Jason

> Typical applications have few (probably only one) listen sockets or
> POSIX mqueues; so I would rather use dedicated threads to issue
> blocking syscalls (accept4 or mq_timedreceive).
> 
> Making blocking syscalls allows exclusive wakeups to avoid thundering
> herds.
> 
>> How do you think, would it be real to implement this? How about
>> concurrency?
>> Can you please give me some hints from which points in code to start
>> to implement these changes?
> 
> For now, I suggest dedicating a thread in each process to do
> mq_timedreceive/mq_receive, assuming you only have a small amount
> of queues in your system.
> 
> 
> [*] mq_timedreceive may copy a largish buffer which benefits from
>     staying on the same CPU as much as possible.
>     Contrary, accept4 only creates a client socket.  With a C10K+
>     socket server (e.g. http/memcached/DB), a typical new client
>     socket spends a fair amount of time idle.  Thus I don't believe
>     memory locality inside the kernel is much concern when there's
>     thousands of accepted client sockets.
> 
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